JP1998-221395A issued in 1998 by the Japan Patent Office discloses a ground fault detector diagnostic apparatus including an electric circuit for electrically connecting a battery and a vehicle travel motor, a ground fault detector for detecting a ground fault in the electric circuit and a ground circuit capable of grounding the electric circuit and a vehicle body. This ground fault detector diagnostic apparatus causes a temporary ground fault of the electric circuit by the ground circuit and diagnoses a failure and the like of the ground fault detector based on whether or not the ground fault detector detects this pseudo-ground fault.